Catheter sensor systems

ABSTRACT

A medical system including a catheter or other flexible structure uses a combination of two different sensor systems to measure a pose of a section or tip of the catheter or flexible structure. The first sensor system, which may include an electromagnetic sensor, can measure a pose of a first section of the catheter, and the second sensor system can measure a pose of the second section relative to the first section. The second sensor system may employ a technology such as fiber shape sensing that permits the second section to be smaller than the first section, so that a distal tip of the catheter or flexible structure can fit within smaller natural lumens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent document is related to and incorporates by reference thefollowing co-filed patent applications: U.S. patent application Ser. No.13/274,198, entitled “Catheters with Control Modes for InterchangeableProbes”; U.S. patent applicaiton Ser. No. 13/274,208, entitled“Catheters with Control Modes for Interchangeable Probes”; and U.S.patent application Ser. No. 13/274,229, entitled “Vision Probe andCatheter Systems.”

BACKGROUND

Medical devices that navigate body lumens need to be physically smallenough to fit within the lumens. Lung catheters, for example, which maybe used to perform minimally invasive lung biopsies or other medicalprocedures, may need to follow airways that decrease in size as thecatheter navigates branching passages. To reach a target location in alung, a catheter may follow passages having diameters as small as 3 mmor less. Manufacturing a catheter that includes the mechanical andsensor structures suitable for remote or robotic operation and that hasa diameter that is sufficiently small to navigate such small lumens canbe challenging. In particular, one desirable configuration for aremotely operated catheter would provide a tool mounted on a steerablesegment; tendons or pull wires that extend down the length of thecatheter to an external drive system that pulls on the tendons toactuate the tool or steerable segment; lumens for suction and/orirrigation; a vision system for viewing of the target location; andsensors to identify the location of the instrument relative to theanatomy of a patient. Accommodating all of the desired features andelements of a lung catheter or other device that is roboticallycontrolled and has a diameter about 3 mm or less can be difficult.

SUMMARY

In accordance with an aspect of the invention, a robotic catheter systemusing distal feedback can provide a small diameter for a distal tip ofthe catheter and accurate measurements of the pose of the distal tipthrough use of a sensor system including both electromagnetic and fibersensors. In accordance with an aspect of the present invention, a sensorsystem for a catheter has a thicker proximal section containing one ormore electromagnetic (EM) sensors and a thinner distal sectioncontaining a fiber shape sensor. The EM sensor can provide an accuratemeasurement of a base point of the distal section relative to theanatomy of a patient, while the fiber sensor measures the shape of thedistal section extending from the base point. Accordingly, the distalsection of the catheter can be as small as a system using only a fibershape sensor, but the catheter system does not suffer from theinaccuracy that is common to long fiber shape sensors.

One specific embodiment of the invention is a medical system including acatheter, a first sensor system, and a second sensor system. Thecatheter has a first section and a second section with the secondsection being adjacent to the first section. The first sensor system isin the first section and configured for measurement of a pose of thefirst section. The second sensor system is in the second section andconfigured for measurement of a pose of the second section relative tothe first section.

Another embodiment of the invention is a method for sensing a pose of adistal tip of an elongated flexible structure such as a catheter in amedical instrument. The method includes applying a time-varying magneticfield to a space containing at least a portion of the flexiblestructure. An electric signal induced in a coil positioned at a locationalong the flexible structure of the medical instrument can then beanalyzed as part of the pose measure. The location of the coil isseparated from a proximal end and the distal tip of the flexiblestructure. In addition to analysis of the electrical signal from thecoil, a shape of a portion of the flexible structure that extends fromthe location of the coil toward the distal end of the flexible sectionis measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a robotic catheter system in accordance with an embodimentof the invention having multiple control modes.

FIG. 2 shows an embodiment of a steerable segment that can be employedin the system of FIG. 1.

FIGS. 3A and 3B show cross-sectional views of proximal and distalsections of a catheter in accordance with an embodiment of theinvention.

FIG. 4 shows a cross-sectional view of a vision probe that may bedeployed in the catheter of FIGS. 3A and 3B and swapped out for use ofmedical probes in the catheter shown in FIGS. 3A and 3B.

FIG. 5 is a flow diagram of a process for using the catheter system witha removable vision system and multiple control modes.

FIG. 6 is a flow diagram of a catheter control process in a holdingmode.

FIG. 7 shows sensing coils that can be employed in electromagneticsensors in medical systems in some embodiments of the invention.

FIGS. 8A, 8B, 8C, and 8D illustrate alternative configurations forsensor systems in accordance with embodiments of the invention includingelectromagnetic and shape sensors.

FIG. 9 shows cross-sections of a catheter system containing asix-degree-of-freedom EM sensor and a catheter system containing twofive-degree-of-freedom EM sensors.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION

A robotic catheter for use in small lumens such as airways and passagesin the respiratory tract employs combinations of one or more EM sensorsand a fiber shape sensor to provide accurate measurements of the pose ofa small-diameter distal tip. FIG. 1 schematically illustrates a cathetersystem 100 in accordance with one embodiment of the invention. In theillustrated embodiment, catheter system 100 includes a catheter 110, adrive interface 120, control logic 140, an operator interface 150, and asensor system 160.

Catheter 110 is a generally flexible device having one or more lumensincluding a main lumen that can accommodate interchangeable probes suchas described further below. Flexible catheters can be made using abraided structure such as a woven wire tube with inner or outer layersof a flexible or low friction material such as polytetrafluoroethylene(PTFE). In one embodiment, catheter 110 includes a bundle of lumens ortubes held together by a braided jacket and a reflowed (i.e., fused bymelting) jacket of a material such as Polyether Block Amide (Pebax).Alternatively, an extrusion of a material such as Pebax can similarly beused to form multiple lumens in catheter 110. Catheter 110 particularlyincludes a main lumen for interchangeable probe systems and smallerlumens for pull wires and sensor lines. In the illustrated embodiment,catheter 110 has a proximal section 112 attached to drive interface 120and a distal section 114 that extends from the proximal section 112. Anadditional steerable segment 116 (e.g., a metal structure such as shownin FIG. 2 and described further below) can form the distal subsection ofdistal section 114. Pull wires extend from drive system 120 throughproximal section 112 and distal section 114 and connect to steerablesegment 116.

The overall length of catheter 110 may be about 60 to 80 cm or longerwith distal section 114 being about 15 cm long and steerable segment 116being about 4 to 5 cm long. In accordance with an aspect of theinvention, distal section 114 has a smaller diameter than does proximalsection 112 and thus can navigate smaller natural lumens or passages.During a medical procedure, at least a portion of proximal section 112and all of distal section 114 may be inserted along a natural lumen suchas an airway of a patient. The smaller diameter of distal section 114permits use of distal section 114 in lumens that may be too small forproximal section 112, but the larger diameter of distal section 114facilitates inclusion of more or larger structures or devices such aselectromagnetic (EM) sensors 162 that may not fit in distal section 114.

Steerable segment 116 is remotely controllable and particularly has apitch and a yaw that can be controlled using pull wires. Steerablesegment 116 may include all or part of distal section 114 and may besimply implemented as a multi-lumen tube of flexible material such asPebax. In general, steerable segment 116 is more flexible than theremainder of catheter 110, which assists in isolating actuation orbending to steerable segment 116 when drive interface 120 pulls onactuating tendons. Catheter 110 can also employ additional features orstructures such as use of Bowden cables for actuating tendons to preventactuation from bending proximal section 112 (or bending any portion thesection of 114 other than steerable segment 116) of catheter 110. FIG. 2shows one specific embodiment in which steerable segment 116 is madefrom a tube 210 that in catheter 110 of FIG. 1 contains multiple tubesdefining a main lumen for a probe system and smaller lumens foractuation tendons 230 and a shape sensor not shown in FIG. 2. In theillustrated embodiment, tendons 230 are placed 90° apart surroundinglumen 312 to facilitate steering catheter 110 in pitch and yawdirections defined by the locations of tendons 230. A reflowed jacket,which is not shown in FIG. 2 to better illustrate the internal structureof steerable segment 116, may also cover tube 210. As shown in FIG. 2,tube 210 is cut or formed to create a series of flexures 220. Tendons230 connect to a distal tip 215 of steerable segment 116 and extend backto a drive interface 120. Tendons 230 can be coated or uncoated, singlefilament or multi strand wires, cables, Bowden cables, hypotubes, or anyother structures that are able to transfer force from drive interface120 to distal tip 215 and limit bending of proximal section 112 whendrive interface 120 pulls on tendons 230. Tendons 230 can be made of anymaterial of sufficient strength including but not limited to a metalsuch as steel or a polymer such as Kevlar. In operation, pulling harderon any one of tendons 230 tends to cause steerable segment 116 to bendin the direction of that tendon 230. To accommodate repeated bending,tube 210 may be made of a material such as Nitinol, which is a metalalloy that can be repeatedly bent with little or no damage.

Drive interfaces 120 of FIG. 1, which pulls on tendons 230 to actuatesteerable segment 116, includes a mechanical system or transmission 124that converts the movement of actuators 122, e.g., electric motors, intomovements of (or tensions in) tendons 230 that run through catheter 110and connect to steerable segment 116. (Push rods could conceivably beused in catheter 110 instead of tendons 230 but may not provide adesirable level of flexibility.) The movement and pose of steerablesegment 116 can thus be controlled through selection of drive signalsfor actuators 122 in drive interface 120. In addition to manipulatingtendons 230, drive interface 120 may also be able to control othermovement of catheter 110 such as range of motion in an insertiondirection and rotation or roll of the proximal end of catheter 110,which may also be powered through actuators 122 and transmission 124.Backend mechanisms or transmissions that are known for flexible-shaftinstruments could in general be used or modified for drive interface120. For example, some known drive systems for flexible instruments aredescribed in U.S. Pat. App. Pub. No. 2010/0331820, entitled “CompliantSurgical Device,” which is hereby incorporated by reference in itsentirety. Drive interface 120 in addition to actuating catheter 110should allow removal and replacements of probes in catheter 110, so thatthe drive structure should be out of the way during such operations.

A dock 126 in drive interface 120 can provide a mechanical couplingbetween drive interface 120 and catheter 110 and link actuation tendons230 to transmission 124. Dock 126 may additionally contain an electronicor optical system for receiving, converting, and/or relaying sensorsignals from portions of sensor system 160 in catheter 110 and containan electronic or mechanical system for identifying the probe or the typeof probe deployed in catheter 110.

Control logic 140 controls the actuators in drive interface 120 toselectively pull on the tendons as needed to actuate and steer steerablesegment 116. In general, control logic 140 operates in response tocommands from a user, e.g., a surgeon or other medical personnel usingoperator interface 150, and in response to measurement signals fromsensor system 160. However, in holding modes as described further below,control logic 140 operates in response to measurement signals fromsensor system 160 to maintain or acquire a previously identified workingconfiguration. Control logic 140 may be implemented using a generalpurpose computer with suitable software, firmware, and/or interfacehardware to interpret signals from operator interface 150 and sensorsystem 160 and to generate control signals for drive interface 120.

In the illustrated embodiment, control logic 140 includes multiplemodules 141, 142, 143, and 144 that implement different processes forcontrolling the actuation of catheter 110. In particular, modules 141,142, 143, and 144 respectively implement a position stiffening mode, anorientation stiffening mode, a target position mode, and a target axialmode, which are described further below. A module 146 selects whichcontrol process will be used and may base the selection on user input,the type or status of the probe deployed in catheter 110, and the taskbeing performed. Control logic 140 also includes memory storingparameters 148 of a working configuration of steerable segment 116 thatis desired for a task, and each of the modules 141, 142, 143, and 144can use their different control processes to actively maintain or holdthe desired working configuration.

Operator interface 150 may include standard input/output hardware suchas a display, a keyboard, a mouse, a joystick, or other pointing deviceor similar I/O hardware that may be customized or optimized for asurgical environment. In general, operator interface 150 providesinformation to the user and receives instructions from the user. Forexample, operator interface 150 may indicate the status of system 100and provide the user with data including images and measurements made bysystem 100. One type of instruction that the user may provide throughoperator interface 150, e.g., using a joystick or similar controller,indicates the desired movement or position of steerable segment 116, andusing such input, control logic 140 can generate control signals foractuators in drive interface 120. Other instructions from the user can,for example, select an operating mode of control logic 140.

Sensor system 160 generally measures a pose of steerable segment 116. Inthe illustrated embodiment, sensor system 160 includes EM sensors 162and a shape sensor 164. EM sensors 162 include one or more conductivecoils that may be subjected to an externally generated electromagneticfield. Each coil of EM sensors 162 then produces an induced electricalsignal having characteristics that depend on the position andorientation of the coil relative to the externally generatedelectromagnetic field. In an exemplary embodiment, EM sensors 162 areconfigured and positioned to measure six degrees of freedom, e.g., threeposition coordinates X, Y, and Z and three orientation angles indicatingpitch, yaw, and roll of a base point. The base point in system 100 is ator near the end of proximal section 112 and the start of distal section114 of catheter 110. Shape sensor 164 in the exemplary embodiment of theinvention includes a fiber grating that permits determination of theshape of a portion of catheter 110 extending from the base point, e.g.,the shape of distal section 114 or steerable segment 116. Such shapesensors using fiber gratings are further described in U.S. Pat. No.7,720,322, entitled “Fiber Optic Shape Sensor,” which is herebyincorporated by reference in its entirety. An advantage of theillustrated type of sensor system 160 is that EM sensors 162 can providemeasurements relative to the externally generated magnetic field, whichcan be calibrated relative to a patient's body. Thus, system 160 can useEM sensors 162 to reliably measure the position and orientation of abase point for shape sensor 164, and shape sensor 164 need only provideshape measurement for a relatively short distance. Additionally, distalsection 114 only contains shape sensor 164 and may have a diameter thatis smaller than the diameter of proximal section 112. More generally,sensor system 160 need only be able to measure the pose of steerablesegment 116, and other types of sensors could be employed.

FIGS. 3A and 3B respectively show cross-sections of the proximal anddistal sections 112 and 114 of catheter 110 in one embodiment of theinvention. FIG. 3A shows an embodiment of catheter 110 having a body 310that includes a main lumen 312 for a vision or medical probe, lumens 314containing tendons 230, lumens 316 containing EM sensors 162 orassociated signal wires, and a lumen 318 containing a fiber shape sensor164. Main lumen 312, tendon lumens 314, and a shape sensor lumen 318extend into distal section 114 as shown in FIG. 3B, but lumens 316 forEM sensors 162 are not needed in distal section 114 because EM sensors162 are only in proximal section 112. Accordingly, distal section 114can be smaller than proximal section 112 particularly because the lumen318 for fiber shape sensor 164 fits between two lumens 314 for pullwires and does not negatively affect the outside diameter of distalsection 114. In an exemplary embodiment, body 310 in proximal section112 has an outer diameter of about 4 mm (e.g., in a range from 3 to 6mm) and provides main lumen 312 with a diameter of about 2 mm (e.g., ina range from 1 to 3 mm) and in distal section 114 has an outer diameterof about 3 mm (e.g., in a range from 2 to 4 mm) while maintaining thediameter of main lumen 312 at about 2 mm. A smooth taper (as shown inFIG. 1) or an abrupt step in body 310 can be used at the transition fromthe larger diameter of proximal section 112 to the smaller diameter ofdistal section 114.

The specific dimensions described in above are primarily for a catheterthat accommodates probes having a diameter of 2 mm, which is a standardsize for existing medical tools such as lung biopsy probes. However,alternative embodiments of the invention could be made larger or smallerto accommodate medical probes with a larger or smaller diameter, e.g., 1mm diameter probes. A particular advantage of such embodiments is that ahigh level of functionality is provided in a catheter with relativesmall outer diameter when compared to the size of probe used in thecatheter.

FIGS. 3A and 3B also show a sheath 360 that may be employed betweencatheter body 310 and a probe in main lumen 312. In one embodiment ofcatheter 110, sheath 360 is movable relative to body 310 and can beextended beyond the end of steerable segment 116. This may beadvantageous in some medical procedures because sheath 360 is evensmaller than distal section 114 and therefore may fit into smallernatural lumens or passages. For example, if catheter 110 reaches abranching of lumens that are too small to accommodate steerable segment116, steerable segment 116 may be pointed in the direction of thedesired branch, so that sheath 360 can be pushed beyond the end ofsteerable segment 116 and into that branch. Sheath 360 could thusreliably guide a medical probe into the desired branch. However, sheath360 is passive in that it is not directly actuated or steerable. Incontrast, distal section 114 accommodates actuation tendons 230 thatconnect to steerable segment 116 and can be manipulated to steer or posesteerable segment 116. In some medical applications, the active controlof steerable segment 116 is desirable or necessary during a medicalprocedure, and passive sheath 360 may not be used in some embodiments ofthe invention.

Main lumen 312 is sized to accommodate a variety of medical probes. Onespecific probe is a vision probe 400 such as illustrated in FIG. 4.Vision probe 400 has a flexible body 410 with an outer diameter (e.g.,about 2 mm) that fits within the main lumen of catheter 110 and withmultiple inner lumens that contain the structures of vision probe 400.Body 410 may be formed using an extruded flexible material such as Pebaxor another polymer, which allows creation of multiple lumens and thinwalls for maximal utility in minimal cross-sectional area. A multi-lumenextrusion also neatly organizes the location of the components. Thelength of body 410 may optionally include a combination of twomulti-lumen extrusions, for example, a distal extrusion “butt-welded” toa proximal extrusion. This may be done, for example, so that theproximal or distal extrusion has desired shape, e.g., a clover-leaf oroval outside shape, to mate with a complementary keying feature incatheter 110. These mating shapes or keying structures can prevent theprobe from rotating within the catheter and assure a known orientationof camera 320 relative to catheter 110.

In the illustrated embodiment, the structure of vision probe 400includes a CMOS camera 420, which is at the distal end of the probe andconnected through one or more signal wires (not shown) that extend alongthe length of vision probe 400, e.g., to provide a video signal tocontrol logic 140 or operator interface 150 as shown in FIG. 1.Alternatively, a fiber bundle imaging system could be employed, but CMOScameras 420 can typically provide images of higher quality than can beachieved with fiber bundle imaging systems. Vision probe 400 alsoincludes illumination fibers 430 that surround camera 420 and providelight for imaging within a body lumen. In an exemplary embodiment,illumination fibers 430 are made of a flexible material such as plastic,which tends to be more flexible than glass fibers. Oblong fluid ports440 are provided in body 410 for suction and irrigation that may beuseful, for example, for rinsing of a lens of camera 420. Fluid ports440 can also be used for delivering drugs, e.g., for numbing, beforevision probe 400 is removed from catheter 110 and replaced with a biopsyprobe. Although the illustrated embodiment of vision probe 400 includesmultiple fluid ports 440, a single fluid port could be used for bothirrigation and suction, and vision probe 400 could alternatively haveonly a single fluid port to save space. Vision probe 400 mayadditionally include an electromagnetic sensor (not shown) embedded justproximally to CMOS camera 420 to provide additional pose informationabout the tip of vision probe 400.

Vision probe 400 is adapted to be inserted or removed from catheter 110while catheter 110 is in use for a medical procedure. Accordingly,vision probe 400 is generally free to move relative to catheter 110.While movement relative to catheter 110 is necessary or desirable duringinsertion or removal of vision probe 400, the orientation of a visionprobe 400 (and some medical probes) may need to be known for optimal oreasier use. For example, a user viewing video from vision probe 400 andoperating a controller similar to a joystick to steer catheter 110generally expects the directions of movement of the controller tocorrespond to the response of steerable segment 116 and the resultingchange in the image from vision probe 400. Operator interface 150 needs(or at least can use) information on the orientation of vision probe 400relative to tendons 230 in order to provide a consistency in directionsused in the user interface. In accordance with an aspect of theinvention, a keying system (not shown) can fix vision probe 400 into aknown orientation relative to catheter 110 and tendons 230. The keyingsystem may, for example, be implemented through the shape of a proximalor distal section of probe 400 or include a spring, fixed protrusion, orlatch on vision probe 400 or steerable segment 116 and a complementarynotch or feature in steerable segment 116 or vision probe 400.

Vision probe 400 is only one example of a probe system that may bedeployed in catheter 110 or guided through catheter 110 to a work site.Other probe systems that may be used include, but are not limited to,biopsy forceps, biopsy needles, biopsy brushes, ablation lasers, andradial ultrasound probes. In general, catheter 110 can be used withexisting manual medical probes that are commercially available frommedical companies such as Olympus Europa Holding GmbH.

The catheter system 100 of FIG. 1 can be used in procedures that swap avision probe and a medical probe. FIG. 5 is a flow diagram of oneembodiment of a process 500 for using the catheter system 100 of FIG. 1.In process 500, vision probe 400 is deployed in catheter 110 in step510, and catheter 110 is inserted along a path including a natural lumenof a patient. For example, for a lung biopsy, steerable segment 116 ofcatheter 110 may be introduced through the mouth of a patient into therespiratory tract of the patient. Vision probe 400 when fully deployedin catheter 110 may fit into a keying structure that keeps vision probe400 in a desired orientation at or even extending beyond steerablesegment 116 to provide a good forward view from steerable segment 116 ofcatheter 110. As noted above, steerable segment 116 of catheter 110 issteerable, and vision probe 320 can provide video of the respiratorytract that helps a user when navigating catheter 110 toward a targetwork site. However, use of vision probe 400 during navigation is notstrictly necessary since navigation of catheter 110 may be possibleusing measurements of sensor system 160 or some other system with orwithout vision probe 400 being deployed or used in catheter 110. Thepath followed to the work site may be entirely within natural lumenssuch as the airways of the respiratory track or may pierce and passthrough tissue at one or more points.

When steerable segment 116 reaches the target work site, vision probe400 can be used to view the work site as in step 530 and to posesteerable segment 116 for performance of a task at the target work siteas in step 540. Posing of steerable segment 116 may use images or visualinformation from vision probe 400 and measurements from sensor system160 to characterize the work site and determine the desired workingconfiguration. The desired working configuration may also depend on thetype of tool that will be used or the next medical task. For example,reaching a desired working configuration of catheter 110 may bring thedistal tip of steerable segment 116 into contact with tissue to betreated, sampled, or removed with a medical tool that replaces visionprobe 400 in catheter 110. Another type of working configuration maypoint steerable segment 116 at target tissue to be removed using anablation laser. For example, tissue could be targeted in one or more 2Dcamera views while vision probe 400 is still in place in catheter 110,or target tissue can be located on a virtual view of the work site usingpre-operative 3D imaging data together with the position sensingrelative to patient anatomy. Still another type of working configurationmay define a line for the insertion of a needle or other medical toolinto tissue, and the working configuration includes poses in which thedistal tip of steerable segment 116 is along the target line. Ingeneral, the desired working configuration defines constraints on theposition or the orientation of the distal tip of steerable segment 116,and the shape of more proximal sections of catheter 110 is not similarlyconstrained and may vary as necessary to accommodate the patient.

Step 550 stores in memory of the control logic parameters that identifythe desired working configuration. For example, the position of a distaltip or target tissue can be defined using three coordinates. A targetline for a need can be defined using the coordinates of a point on theline and angles indicating the direction of the line from that point. Ingeneral, control logic 120 uses the stored parameters that define thedesired working configuration when operating in a holding mode thatmaintains steerable segment 116 of catheter 110 in the desired workingconfiguration as described further below.

Step 560 selects and activates a holding mode of the catheter systemafter the desired working configuration has been established andrecorded. Control logic 140 for catheter 110 of FIG. 1 may have one ormore modules 141, 142, 143, and 144 implementing multiple stiffeningmodes that may be used as holding modes when the desired configurationof steerable segment 116 has fixed constraints. The available controlmodes may include one or more of the following.

1.) A position stiffness mode compares the position of the distal tip ofsteerable segment 116 as measured by sensor system 160 to a desired tipposition and controls the actuators to minimize the difference indesired and measured tip positions. The position stiffness mode mayparticularly be suitable for general manipulation tasks in which theuser tries to precisely control the position of the tip and forsituations where the distal tip contacts tissue.

2.) An orientation stiffness mode compares the measured orientation orpointing direction of the distal tip to a desired pointing direction ofthe distal tip and controls the actuators to minimize the difference indesired and actual tip pointing direction. This orientation stiffeningthat may be suitable, e.g., when controlling an imaging device such asvision probe 400 attached steerable segment 116, in which case theviewing direction is kept as desired, while the exact position ofsteerable segment 116 may be less important.

3.) A target position stiffness mode uses a combination of the measuredtip position and pointing direction to control catheter 110 to alwayspoint the distal tip of steerable segment 116 towards a specified targetpoint some distance in front of steerable segment 116. In case ofexternal disturbances, control logic 140 may control the actuators toimplement this target position stiffening behavior, which may besuitable, e.g., when a medical probe inserted though the cathetercontains an ablation laser that should always be aimed at a targetablation point in tissue.

4.) A target axial motion stiffness mode uses a combination of themeasured tip position and pointing direction to ensure that the distaltip of steerable segment 116 is always on a line in space and has apointing direction that is also along that line. This mode can beuseful, e.g., when inserting a biopsy needle along a specified line intotissue. Tissue reaction forces could cause the flexible section ofcatheter 110 to bend while inserting the needle, but this controlstrategy would ensure that the needle is always along the right line.

The selection of a mode in step 560 could be made through manualselection by the user, based on the type of probe that is being used(e.g., grasper, camera, laser, or needle) in catheter 110, or based onthe activity catheter 110 is performing. For example, when a laser isdeployed in catheter 110, control logic 120 may operate in positionstiffness mode when the laser deployed in catheter 110 is off andoperate in target position stiffness mode to focus the laser on adesired target when the laser is on. When “holding” is activated,control logic 140 uses the stored parameters of the workingconfiguration (instead of immediate input from operator interface 150)in generating control signals for drive interface 120.

The vision probe is removed from the catheter in step 570, which clearsthe main lumen of catheter 110 for the step 580 of inserting a medicalprobe or tool through catheter 110. For the specific step order shown inFIG. 5, control logic 140 operates in holding mode and maintainssteerable segment 116 in the desired working configuration while thevision system is removed (step 570) and the medical probe is inserted(step 580). Accordingly, when the medical probe is fully deployed, e.g.,reaches the end of steerable segment 116, the medical probe will be inthe desired working configuration, and performance of the medical taskas in step 590 can be then performed without further need or use of theremoved vision probe. Once the medical task is completed, the cathetercan be taken out of holding mode or otherwise relaxed so that themedical probe can be removed. The catheter can then be removed from thepatient if the medical procedure is complete, or the vision or anotherprobe can be inserted through the catheter if further medical tasks aredesired.

In one alternative for the step order of process 500, catheter 110 maynot be in a holding mode while the medical probe is inserted but can beswitched to holding mode after the medical probe is fully deployed. Forexample, catheter 110 may be relaxed or straightened for easy remove ofvision probe 400 (step 570) and insertion of the medical probe (step580). Once holding mode is initiated, e.g., after insertion of themedical probe, control logic 140 will control the drive interface 130 toreturn steerable segment 116 to the desired working configuration ifsteerable segment 116 has moved since being posed in the desired workingconfiguration. Thereafter, control logic 140 monitors the pose ofsteerable segment 116 and actively maintains steerable segment 116 inthe desired working configuration while the medical task is performed instep 590.

FIG. 6 shows a flow diagram of a process 600 of a holding mode that canbe implemented in control logic 140 of FIG. 1. Process 600 begins instep 610 with receipt of measurement signals from sensor system 160. Theparticular measurements required depend on the type of holding modebeing implemented, but as an example, the measurements can indicateposition coordinates, e.g., rectangular coordinates X, Y, and Z, of thedistal tip of steerable segment 116 and orientation angles, e.g., anglesθ_(X), θ_(Y), and θ_(Z) of a center axis of the distal tip of steerablesegment 116 relative to coordinate axes X, Y, and Z. Other coordinatesystems and methods for representing the pose of steerable segment 116could be used, and measurements of all coordinates and direction anglesmay not be necessary. However, in an exemplary embodiment, sensor system160 is capable of measuring six degrees of freedom (DoF) of the distaltip of steerable segment 116 and of providing those measurements tocontrol logic 140 in step 610.

Control logic 140 in step 620 determines a desired pose of steerablesegment 116. For example, control logic 140 can determine desiredposition coordinates, e.g., X′, Y′, and Z′, of the end of steerablesegment 116 and desired orientation angles, e.g., angles θ′_(X), θ′_(Y),and θ′_(Z) of the center axis of steerable segment 116 relative tocoordinate axes X, Y, and Z. The holding modes described above generallyprovide fewer than six constraints on the desired coordinates. Forexample, position stiffness operates to constrain three degrees offreedom, the position of the end of steerable segment 116 but not theorientation angles. In contrast, orientation stiffness mode constrainsone or more orientation angles but not the position of end of steerablesegment 116. Target position stiffness mode constrains four degrees offreedom, and axial stiffness mode constrains five degrees of freedom.Control logic 610 can impose further constraints to select one of set ofparameters, e.g., X′, Y′, and Z′ and angles θ′_(X), θ′_(Y), and θ′_(Z),that provides the desired working configuration. Such furtherconstraints include but are not limited to mechanical constraintsrequired by the capabilities of steerable segment 116 and of catheter110 generally and utilitarian constraints such as minimizing movement ofsteerable segment 116 or providing desired operating characteristicssuch as smooth, non-oscillating, and predictable movement withcontrolled stress in catheter 110. Step 620 possibly includes justkeeping a set pose steerable segment 116 by finding smallest movementfrom the measured pose to a pose satisfying the constraints, e.g.,finding the point on the target line closest to the measure position foraxial motion stiffness or finding some suitable pose from registeredpre-op data that is close to the current pose.

Control logic 140 in step 630 uses the desired and/or measured poses todetermine corrected control signals that will cause drive interface 120to move steerable segment 116 to the desired pose. For example, themechanics of catheter 110 and drive interface 120 may permit developmentof mappings from the desired coordinates X′, Y′, and Z′ and anglesθ′_(X), θ′_(Y), and θ′_(Z) to actuator control signals that provide thedesired pose. Other embodiments may use differences between the measuredand desired pose to determine corrected control signals. In general, thecontrol signals may be used not only to control actuators connectedthrough tendons to steerable segment 116 but may also control (to somedegree) insertion or roll of catheter 110 as a whole.

A branch step 650 completes a feedback loop by causing process 600 toreturn to measurement step 610 after control system 140 applies newcontrol signals drive interface 120. The pose of distal tip is thusactively monitored and controlled according to fixed constraints as longas control system 120 remains in the holding mode. It may be noted,however, that some degrees of freedom of steerable segment 116 may notrequire active control. For example, in orientation stiffness mode,feedback control could actively maintain pitch and yaw of steerablesegment 116, while the mechanical torsional stiffness of catheter 110 isrelied on hold the roll angle fixed. However, catheter 110 in generalmay be subject to unpredictable external forces or patient movement thatwould otherwise cause catheter 110 to move relative to the work site,and active control as in process 600 is needed to maintain or hold thedesired working configuration.

The sensor system 160 of a catheter 100 as noted above can employ bothan EM sensor 162 and a fiber shape sensor 164. EM sensors or trackersare state-of-the-art position and orientation sensors that combine highglobal accuracy with small package size (e.g., about 1×10 mm). EMsensors are commercially available from companies such as AscensionTechnology Corporation and Northern Digital Inc. Shape sensingtechnology, which may be used in the above described embodiments,commonly employ reflections and interference within an optical fiber tomeasure the shape along the length of the optical fiber. This shapesensing technology is good for giving 6-DoF relative measurementsbetween two points along the fiber as well as measuring bend angles ofcontrollable joints or providing full three-dimensional shapeinformation. A typical fiber shape sensor of this type may have adiameter of about 0.2 mm, which is considerably smaller than a typicalEM sensor.

FIG. 7 illustrates three different types of sensing coils 710, 720, and730 that could be used in an EM sensor. In operation, the sensing coil,e.g., coil 710, in the catheter or other device is placed in awell-controlled magnetic field that an external EM generator produces.The EM generator typically has the form of a square or cylindrical boxof 20-60 cm wide and several cm thick and may have a fixed positionrelative to the patient. The magnetic field produced by the generatorvaries in time and induces a voltage and electric current in the sensingcoil 710. U.S. Pat. No. 7,197,354, entitled “System for Determining thePosition and Orientation of a Catheter”; U.S. Pat. No. 6,833,814,entitled “Intrabody Navigation System for Medical Applications”; andU.S. Pat. No. 6,188,355, entitled “Wireless Six-Degree-of-FreedomLocator” describe the operation of some EM sensor systems suitable forin medical environment and are hereby incorporated by reference in theirentirety. U.S. Pat. No. 7,398,116, entitled “Methods, Apparatuses, andSystems useful in Conducting Image Guided Interventions,” U.S. Pat. No.7,920,909, entitled “Apparatus and Method for Automatic Image GuidedAccuracy Verification,” U.S. Pat. No. 7,853,307, entitled “Methods,Apparatuses, and Systems Useful in Conducting Image GuidedInterventions,” and U.S. Pat. No. 7,962,193, entitled “Apparatus andMethod for Image Guided Accuracy Verification” further describe systemsand methods that can use electromagnetic sensing coils in guidingmedical procedures and are also incorporated by reference in theirentirety. In general, the induced voltage in a sensing coil depends ontime derivative the magnetic flux, which in turn depends on the strengthof the magnetic field and the direction of the magnetic field relativeto a normal to the areas of loops in the coil. The field generator canvary the direction and magnitude of the magnetic field in a systematicmanner that enables at least partial determination of the pose of coil710 from the induced electric signal. Up to five degrees of freedom canbe determined using a sensor 162 containing a single sensing coil 710.However, sensing coil 710 is cylindrically symmetric, so that a rollangle, i.e., an angle indicating orientation about a normal 712 to theinductive areas of coil 710, cannot be determined. Only the position andthe pointing direction can be determined using a single coil 710. Evenso, a 5-Degree-of-Freedom (5-DoF) sensor containing a single sensingcoil 710 is useful in many medical systems. In particular, themechanical shape of a typical sensing coil (long and slender) fits wellwith the mechanical shape of minimally invasive medical tools, and ifthe tool is rotationally symmetrical (e.g. in the case of a needle orlaser fiber), the roll angle is not relevant.

A robotic control catheter such as catheter 110 may need a 6-DoFmeasurement including a measurement of the roll angle so that thepositions of actuating tendons are known. If measurement of the rollangle is of interest, two 5-DoF EM sensors can be combined to create a6-DoF EM sensor. One specific configuration of a 6-DoF EM sensor usestwo coils such as 710 with the inductive areas of two coils havingnormal vectors that are askew, e.g., perpendicular to each other. Moregenerally, the two coils need to be arranged so that the normal vectorsto inductive areas are not along the same axis, and larger anglesbetween the normal vectors generally provide better measurementaccuracy. Coils 720 and 730 illustrate how a coil 720 or 730 that mayhave wire loops with a normal 722 or 732 that is at a non-zero angle tothe axes of a cylinder containing the coil 720 or 730. Coils 720 and 730can thus be oriented along the same direction, e.g., along the length ofa catheter or other medical tool, and still be used to measure sixdegrees of freedom.

FIG. 8A shows a configuration of a catheter system 810 having a proximalsection 812 containing a 6-DoF EM sensor 820 and a distal section 814containing a fiber shape sensor 822. EM sensor 820 terminates at or neara distal end of proximal section 812. Accordingly, distal section 814can have a diameter (e.g., about 3 mm to accommodate a probe diameter ofabout 2 mm) that is smaller than the diameter (e.g., about 4 mm) ofproximal section 812 because EM sensor 820 does not extend into distalsection 814. The pose of distal tip of section 814 can be determinedusing EM sensor 820 to measure or determine the global position andorientation of a point along shape sensor 822 and using shape sensor 822to determine the shape of distal section 814 extending from the measuredpoint. The accuracy of shape sensor 822 can be relatively high becauseshape sensor 822 only needs to measure the shape of a relatively shortsection 814 of catheter 810, rather than the entire length of catheter810. For example, in one case, the accuracy of the position measurementfor the distal tip of section 814 is a function of the position andorientation accuracy of the EM sensor 820 (typically about 1 mm and 0.01radians respectively) and the position accuracy of the shape sensor(0.15% of the length of section 814). If 6-DoF EM sensor 820 is about115 mm away from the distal tip, the typical tip position accuracy wouldbe about 2.5 mm.

FIG. 8B shows a catheter 830 that uses two 5-DoF EM sensors 840 and 841in a proximal section 832 to measure six degrees of freedom of a basepoint along a shape sensor 842 that extends into a distal section 834 ofcatheter 830. Coils of EM sensors 840 and 841 are within the samecross-section of proximal section 832 and therefore are rigidly fixedrelative to each other. EM sensors 840 and 841 can also contain sensingcoils such as coils 720 and 730 having wire loops with differentorientations to measure different degrees of freedom of a point alongshape sensor 842. The roll angle can thus be determined using the twomeasured pointing directions of sensors 840 and 841 to define areference frame. The use of 5-DoF sensors 840 and 841 may allow areduction in the diameter of proximal section 832. In particular, 6-DoFEM sensors that are available commercially generally have diameters thatare larger than the diameters of similar 5-DoF EM sensors. In accordancewith an aspect of the current invention, the diameter of a catheter maybe decreased through use of 5-DoF EM sensors. FIG. 9, for example,illustrates how a distal section 832 of catheter 830 can accommodate two5-DoF EM sensors 840 and 841 and a main lumen 910 within a circularcross-sectional area that is smaller than the area of distal section 812of catheter 810. In particular, section 812 is larger because section812 must accommodate the main lumen and a 6-DoF sensor that has a largerdiameter than do 5-DoF sensors 840 and 841.

FIG. 8C shows an embodiment of the invention using a sensor system thatmay allow a proximal section 852 of catheter 850 to be even smaller byusing two 5-DoF EM sensors 860 and 861 that are separated along thelength of the proximal section 852. Accordingly, only one 5-DoF EMsensor 860 or 861 needs to be accommodated within the cross-section ofproximal section 852. However, since distal section 852 is flexible andmay be bent when in use, EM sensors 860 and 861 are not rigidly fixedrelative to each other, and a shape sensor 862 is used to measure theshape of the portion of proximal section 852 between EM sensors 861 and860 and the relative orientation of EM sensors 860 and 861. The shapemeasurement between EM sensors 861 and 860 indicates the position andorientation of sensor 860 relative to sensor 861, and the relativeconfiguration is needed for determination of a 6-DoF measurement fromthe two 5-DoF measurement. Shape sensor 862 also measures the shape ofdistal section 854, which indicates the position and orientation of thedistal tip relative to the global position and orientation measurementsdetermined using EM sensors 860 and 861.

FIG. 8D shows yet another catheter 870 using two 5-DoF EM sensors 880and 881 that are separate along the length of catheter 870. The sensingsystem in catheter 870 of FIG. 8D differs from the sensing systems ofFIGS. 8A, 8B, and 8C in that one EM sensor 880 is located in a proximalsection 872 of catheter 870 and the other EM sensor 881 is located in adistal section 874 of catheter 870. Accordingly, distal section 874 mustbe large enough to include sensor 881, but still may allow a reductionin the diameter of catheter 870 when compared to a catheter having a6-DoF EM sensor at a distal tip.

The use of two 5-DoF EM sensors in embodiments of FIGS. 8B, 8C, and 8Dprovides more information than is strictly required for a 6-DoFmeasurement. In accordance with a further aspect of the invention, oneof the two 5-DoF EM sensors in catheter 830, 850, or 870 of FIG. 8B, 8C,or 8D could be replaced with another type of sensor that may not measurefive degrees of freedom. For example, an accelerometer could be employedin place of one of the two EM sensors and provide a measurement of thedirection of gravity, i.e., down. Provided that the symmetry axis of the5-DoF sensor is not vertical, the combination of the measurements of a5-DoF sensor and a measurement of the orientation relative to thevertical direction is sufficient to indicate measurements for sixdegrees of freedom.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. Various adaptationsand combinations of features of the embodiments disclosed are within thescope of the invention as defined by the following claims.

What is claimed is:
 1. A medical system comprising: a catheter having afirst section and a second section, the second section adjacent to thefirst section, wherein the first section defines a first longitudinalcatheter axis and the second section defines a second longitudinalcatheter axis; an electromagnetic sensor system in the first sectionincluding: a first electromagnetic sensor in a first lumen portionextending along a longitudinal sensor axis in parallel with the firstlongitudinal catheter axis at a proximal end of the first section, and asecond electromagnetic sensor in a second lumen portion extending alongthe longitudinal sensor axis in parallel with the first longitudinalcatheter axis at a distal end of the first section, the distal end ofthe first section being adjacent to the second section, wherein thefirst section between the proximal and distal ends flexibly couples thefirst and second electromagnetic sensors so that the firstelectromagnetic sensor is movable with respect to the secondelectromagnetic sensor, wherein the first electromagnetic sensorincludes a first signal wire coupled to a first coil extending withinthe first lumen portion along the longitudinal sensor axis and thesecond electromagnetic sensor includes a second signal wire coupled to asecond coil extending within the second lumen portion along thelongitudinal sensor axis; and a fiber shape sensor system that extendsthrough the first section between and along the first electromagneticsensor and the second electromagnetic sensor, the fiber shape sensorextending through the second section, the fiber shape sensor systembeing configured for measurement of a pose of the second sectionrelative to the distal end of the first section and measurement of arelative orientation between the first electromagnetic sensor and thesecond electromagnetic sensor.
 2. The system of claim 1, wherein thesecond section is narrower than the first section.
 3. The system ofclaim 2, wherein the first longitudinal catheter axis is offset from thesecond longitudinal catheter axis.
 4. The system of claim 1, wherein thefirst electromagnetic sensor is configured to measure a position and anorientation of the proximal end of the first section relative to anexternal reference.
 5. The system of claim 4, wherein the secondelectromagnetic sensor is configured to measure a position and anorientation of the distal end of the first section relative to theproximal end of the first section.
 6. The system of claim 1, wherein thefirst electromagnetic sensor and the second electromagnetic sensor arefive-degree-of-freedom sensors and wherein the electromagnetic sensorsystem and the fiber shape sensor system comprise a measurement systemfor determining a six-degree-of-freedom measurement.
 7. The system ofclaim 1, wherein the fiber shape sensor system comprises Bragg gratings.8. The system of claim 1, wherein the first coil has a first magneticaxis oriented at a first angle relative to the first longitudinalcatheter axis; and the second coil has a second magnetic axis that isoriented at a second angle relative to the first longitudinal catheteraxis, the first and second angles being different such that the secondmagnetic axis is askew from the first magnetic axis.
 9. The system ofclaim 1, wherein the fiber shape sensor system is configured to measurea spatial relationship between the first coil and the second coil. 10.The system of claim 1, wherein the first section further comprises anaccelerometer configured to measure a direction of gravity.
 11. A methodfor sensing a pose of a distal tip of an elongated flexible structure ofa medical instrument, the method comprising: applying a time-varyingmagnetic field to a space containing at least a portion of the elongatedflexible structure, the elongated flexible structure defining aninstrument longitudinal axis and comprising a catheter; analyzing anelectric signal induced in a first coil positioned with a first signalwire in a first lumen portion having a sensor longitudinal axis arrangedin parallel with the instrument longitudinal axis at a first locationalong the flexible structure of the medical instrument, wherein thefirst location is separated from a proximal end and a distal end of theflexible structure; and measuring, with a shape sensor, a shape of aportion of the flexible structure of the medical instrument, wherein theportion measured extends from the first location of the first coiltoward the distal end of the flexible structure; measuring, with theshape sensor, a relative orientation between the first coil and a secondcoil, wherein the second coil is positioned with a second signal wire ina second lumen portion along the sensor longitudinal axis arranged inparallel with the instrument longitudinal axis at a second locationalong the flexible structure such that the first coil is flexiblycoupled to and movable with respect to the second coil as the flexiblestructure bends and wherein the shape sensor extends between and alongthe first and second coils; and generating, by control logic, acorrected actuator control signal based on a desired configuration ofthe flexible structure and the measured shape of the portion of theflexible structure extending from the first location of the first coiltoward the distal end of the flexible structure and based on themeasured relative orientation between the first coil and the secondcoil.
 12. The method of claim 11, wherein the catheter comprises: afirst catheter section containing the first and second coils; and asecond catheter section adjacent to the first section and extending tothe distal tip.
 13. The method of claim 11, wherein the control logicdetermines a six-degree-of-freedom measurement from a firstfive-degree-of-freedom measurement obtained with the first coil and asecond five-degree-of-freedom measurement obtained with the second coil.14. The method of claim 11, wherein analyzing the electric signal in thefirst coil comprises determining a global position and orientation at apoint along the shape sensor.
 15. The method of claim 14, determiningthe global position and orientation further comprises using a secondelectrical signal induced in the second coil positioned along theflexible structure.
 16. A medical system comprising: a catheter having afirst section and a second section, the second section adjacent to thefirst section, wherein the catheter defines a longitudinal catheteraxis; a first electromagnetic sensor in a first lumen portion alignedalong a longitudinal sensor axis arranged in parallel with thelongitudinal catheter axis at a first location along the first section;a second electromagnetic sensor in a second lumen portion aligned alongthe longitudinal sensor axis arranged in parallel with the longitudinalcatheter axis at a distal end of the first section, the distal end ofthe first section being adjacent to the second section, wherein thefirst section between the first location and distal end flexibly couplesthe first and second electromagnetic sensors so that the firstelectromagnetic sensor is movable with respect to the secondelectromagnetic sensor, and wherein the first and second electromagneticsensors are five-degree-of-freedom sensors; and a fiber shape sensorsystem that extends through the first section between and along thefirst electromagnetic sensor and the second electromagnetic sensor, thefiber shape sensor extending through the second section, the fiber shapesensor system being configured for measurement of a pose of the secondsection relative to the distal end of the first section and measurementof a relative orientation between the first electromagnetic sensor andthe second electromagnetic sensor; and control logic that receivesmeasurement information from the first and second electromagneticsensors and the fiber shape sensor and that generates a correctedactuator control signal based on a desired configuration of the catheterand a measured shape of the portion of the catheter extending from thefirst location toward the distal end of the first section and based onthe measured relative orientation between the first electromagneticsensor and the second electromagnetic sensor.
 17. The medical system ofclaim 16, wherein the first electromagnetic sensor includes a first coilthat has a first magnetic axis oriented at a first angle relative to thelongitudinal catheter axis; and the second electromagnetic sensorincludes a second coil has a second magnetic axis that is oriented at asecond angle relative to the longitudinal catheter axis, the first andsecond angles being different such that the second magnetic axis isaskew from the first magnetic axis.
 18. The medical system of claim 16,wherein the second section is narrower than the first section and thefirst and second lumen portions are portions of a sensor lumen extendingalong a length of the first section.
 19. The medical system of claim 16,wherein the control logic determines a six-degree-of-freedom measurementfrom a first five-degree-of-freedom measurement from the firstelectromagnetic sensor and a second five-degree-of-freedom measurementfrom the second electromagnetic sensor.